1. Technical Field
The invention relates generally to a method of fabricating a mask using a lithography technique in semiconductor fabrication. Particularly, to an iterative method of refining an integrated circuit (IC) design layout based on optical proximity correction (OPC), where edge fragments are grouped accordingly in non-opposing directional orientations.
2. Background Art
The trend of decreasing conductive feature size and increasing feature density in semiconductor devices diminishes the precision of lithographic printing compromising yield or reliability. As the critical dimension (CD) of conductive features continues to shrink far beyond a submicron scale crossing into or below the wavelength of light, distortions of the aerial image of the IC design to the target layout in the lithographic printing process are unavoidable. Such distortions may lead to overlap of conductive features. To minimize such optical distortions, resolution enhancement techniques (RET) including phase shift and optical proximity correction (OPC) are employed to attain greater precision towards the ideal properties of the lithography process.
To a person skilled in the art, an IC design or layout is composed of numerous polygons made up of edge fragments. To enable compensation of optical distortions, these edge fragments are fragmentized and shifted in small increments. Typically, OPC is divided into model-based (MBOPC) or rule-based (RBOPC). MBOPC applies computationally intensive simulations of an edge placement error determined for every edge in the IC design to compensate for the error through a series of iterations. Due to computational requirements, MBOPC is time consuming. RBOPC extends from manual OPC where alterations of every particular shape of fragmented polygon in an IC design is set according to a pre-determined rule. RBOPC techniques, though faster, strongly depend on empirical knowledge for an accurate correction.
For the MBOPC technique, consideration of the deleterious effects of the edge placement error shift of neighboring edge fragments is not included in the simulations of a particular edge fragment. This poses problems in convergence, accuracy of fit and adhering to mask rule checks (MRC). When an edge fragment shifts simultaneously with neighboring edge fragments, comparison of positions between the moving edge fragments adds to computational complexities because of the interdependency between the edge fragments. Edge placement error (EPE) shifts of surrounding edge fragments may conflict with efforts to minimize the edge placement error of a particular edge fragment. Accuracy is compromised at roughly half a grid point times the sensitivity of the fragment movement to edge placement error (EPE) with the use of a single fragment to a single target edge. A further limitation is the application of mask rule checks (MRC) for ensuring minimal distance between features, minimal spacing externally and minimum distance of a feature (e.g. a minimum line internally). Generally, repair steps to correct any irregularities may be built into the process which follows from MRC. The complexity of a repair step may add to each iteration step on shifting all edges to verify compliance with MRC and refining intervening problems after all edges have moved before continuing on to another iteration step.
Efforts have been made to reduce the limitations of MBOPC with consideration of classifying edges based on orientation angles and cooperation schemes between neighboring edge fragments on a linear combination of the respective edge placement errors of related edge fragments. However, the additional computational steps in these efforts may create further computational complications. Furthermore, there remains the need to address the effect of edge placement error shifts of opposite edge fragments. Some methods proposed have iterated different categories of fragments by characteristics such as line end, corner, horizontal or vertical, but have never separated this into moving non-opposing fragments that is directionally specific at a time, for example, these prior methods have not separated top fragments from bottom fragments or left fragments from right fragments. Additionally, since conventional OPC will move a fragment based only on the EPE of itself and not its neighbors, there may be an accuracy limit of about half a grid point times the sensitivity of the fragment movement.
In view of the foregoing, there is a need in the art for a solution to the problems of the related art.